1. Field of the Invention
The present invention relates to zener diodes and a method of fabricating zener diodes; more particularly, to the fabrication of zener diodes compatible with CMOS technology.
2. Description of the Related Art
Zener diodes are an important part of analog, linear integrated circuits. The performance of zener diodes is determined, in large part, by the junction between the N and P regions which form the diode.
Conventional methods of fabricating zener diodes involve compensation or counter-doping. A P.sup.+ region is provided and then partially masked. N-type dopant ions are implanted in the exposed, i.e., unmasked, portion of the P.sup.+ region to counter-dope the region, thereby providing the N.sup.+ region of the zener diode. This so-called compensation method is satisfactory with CMOS devices having relatively deep junctions, for example, a junction depth of approximately 2 microns. With deep junctions the desired resistivity of the N.sup.+ region, i.e., 15-20 .OMEGA./square, can be achieved with a doping level on the order of 2-3.times.10.sup.20 atoms/cm.sup.3. However, on a relative basis a large amount of doping is required to counter-dope a P.sup.+ region to provide it with N.sup.+ type conductivity characterization.
The current trend is to reduce junction depths in CMOS devices to less than 1 micron to provide better transistor characteristics. Reducing the junction depth requires an increased doping level to achieve the same resistivity of the N.sup.+ region because the same number of dopant ions must be provided in a smaller volume of material. For example, reducing the junction depth from 2.1 microns to 0.7 microns requires a tripling of the doping level from approximately 2-3.times.10.sup.20 atom/cm.sup.3 to approximately 6-9.times.10.sup.20 atoms/cm.sup.3. Zener diodes fabricated by the compensation method with such high doping levels exhibit erratic electrical characteristics. Such zener diodes have a breakdown voltage which was not reproducible and in some cases is as small as `volt. A 1 volt breakdown voltage is less than standard biasing voltages, and renders the device useless as a zener diode.